A monocoque vehicle body is typically fabricated by preparing a number of panel members and frame members by stamp forming steel sheet metal and joining them together by spot welding (see Japanese patent laid open publication No. 2002-302071). In a conventional vehicle body, the impact created by a frontal collision of the vehicle with another vehicle or a fixed structure is transmitted from the bumper to a pair of side front side frames extending in the fore-and-aft direction in a laterally spaced apart relationship. A part of the impact is then transmitted to side sill inners via corresponding outriggers while most of the impact is transmitted to a middle cross member via a right and left floor frame.
In the vehicle body structure disclosed in the aforementioned patent publication, a central part of the floor panel bulges upward to accommodate a fuel tank under it, and the fuel tank is received in a space defined by a front cross member, the middle cross member and floor frames. To the upper surface of the floor panel are attached the front and rear legs of seat frames that support the front seats.
In such a vehicle body structure, because the impact load transmitted by the front side frames is in most part applied to the floor frames, the floor frames are required to have large cross sectional areas and/or made of relatively thick steel sheets so that they add a significant weight to the overall weight of the vehicle body. An increased vehicle weight is obviously not desirable as it adversely affects the fuel economy and driving performance of the vehicle. It has therefore been desired to more effectively distribute the impact load transmitted to the front side frames to frame members other than the floor frames.
Also, in this vehicle body structure, when a side impact is applied to a front part of the vehicle body, the impact load applied from the corresponding side sill inner is transmitted to the front cross member. Therefore, if the fuel tank is located under the floor panel, the front cross member is required to be reinforced by using thicker steel sheets, and this also contributes to the increase in the weight of the vehicle body.
Furthermore, at the time of a frontal impact, the vehicle occupant is thrown forward under an inertia force while restrained by a seat belt. As a result, the front part of the vehicle seat receives a significant downward force, and this may cause a downward deflection of the floor panel to which the front legs of the seat frames are attached. Such a downward deflection of the floor panel must be controlled because the head of the vehicle occupant may be brought too close to the dashboard panel. This necessitates a floor panel having a relative large section modulus and using a relatively thick steel sheet. This again added to the increase in the weight of the vehicle body, and makes the stamping work more difficult.